Extending the collection efficiency of near-infrared (NIR) photons in low-cost, earth abundant semiconductors is an ongoing challenge. For example, silicon (bandgap, Eg=1.1 eV) and cadmium telluride (Eg=1.5 eV), are unable to collect infrared photons that have wavelengths exceeding 1100 and 800 nm respectively. This limitation impacts solar energy conversion, since single junction photovoltaic cells based on these materials are subject to the Shockley-Queisser limit on their efficiencies. A scientific challenge for “Third Generation” photovoltaic technologies is to extract useful energy from all solar wavelengths and surpass the Shockley-Queisser efficiency limit.
In upconversion, two lower energy photons can be combined into a single higher energy photon that lies above the semiconductor bandgap and can be absorbed by the solar cell. This process can lead to substantial gains in overall solar conversion efficiency, for example, from 32% under the Shockley-Queisser limit to 43% under one sun for photovoltaic cells with a bandgap of 1.76 eV. The upconversion process has been realized in organic systems, in which a sensitizer absorbs a long wavelength photon, undergoes intersystem crossing to its triplet state, and then transfers its energy to the triplet state of a second molecule that acts as an emitter. When this happens twice, triplet-triplet annihilation (TTA) can occur, with two triplets fusing into a higher energy singlet state on the emitter. Organic upconversion systems can harvest photons out to 790 nm with demonstrated efficiencies, for example, approximately 1% in the NIR. Unfortunately, it has proved challenging to find photostable, long-lived organic NIR chromophores that will allow organic upconversion systems to boost the efficiency of commercially relevant photovoltaic materials like Si or CdTe. Rare earth glasses have also been used for upconversion, but they have limited spectral coverage and low efficiencies due to the forbidden nature of the optical transitions in lanthanides employed for upconversion. In order to impact current solar energy technologies, a challenge is to extend upconversion materials into the NIR spectral region.
Inorganic nanocrystals (NCs) are relatively photostable chromophores whose bandgap absorption spectra can be tuned from the near ultraviolet to the infrared. Previous reports of triplet energy transfer from organics to semiconductors suggest that it is possible for NC excitons state to exchange energy with molecular triplet states. A good example is the recent demonstration that triplet excitons produced by singlet fission in pentacene and tetracene layers can efficiently transfer their energy to adjacent semiconductor NC layers. Evidence for the reverse process, for example, energy transfer from the NC to the triplet state of an organic molecule, is scarce. Earlier experiments have provided some evidence for this transfer is possible, showing that NCs could sensitize singlet oxygen and that the low-lying triplet state of a naphthyl ligand could act as a trap state for CdSe NCs.
In accordance with an exemplary embodiment, the disclosure demonstrates that NC-organic triplet sensitization is a robust phenomenon that can be optimized through ligand chemistry. In addition, NC-organic triplet sensitization can be used to upconvert photons across the visible and infrared spectral regions. Applications of upconversion materials can include multiphoton imaging, data storage, optical displays, and lighting. This invention also has the potential to extend upconversion into the infrared spectral region, where it could be used to enhance the efficiency of commercially viable photovoltaic materials like CdTe and Si.